A dead-center spring element can be any spring element, in principle. The function thereof determines that a movable part is acted on in the direction of a closed position when the movable part is located between dead center and a closed position, and the movable part is acted on in the direction of a usage position when the movable part is located between dead center and the usage position. In addition, the dead-center spring element holds the movable part in the closed position or in the usage position. The closed position and the usage position can also be referred to, in general, as the first position and the second position or as one position and the other position of the movable part. Dead center is located between the closed position and the open position, but not necessarily at the center between these two positions. The dead center is a point at which the dead-center spring element acts on the movable part neither in one direction nor in the other direction. The dead center can shift depending on a direction of motion of the movable part. The function of a dead-center spring element is achieved, as stated, not by means of a special spring element, but rather by means of mechanics, guidance, deflection, and/or a geometric arrangement of the dead-center spring element and the movable part.
European patent application EP 2 290 183 A1 discloses a box-shaped storage compartment comprising a swiveling lid as a movable part, and a dead-center spring element which swivelably engages at a lever arm of the swiveling lid and swivelably rests on a side wall of the storage compartment, as an abutment. During opening and closing of the swiveling lid, an engagement point of the dead-center spring element moves on a circular path on the lever arm of the swiveling lid about a swivel axis of the swiveling lid. A geometry is selected in such a way that, during opening and closing of the swiveling lid, the engagement point of the dead-center spring element first approaches the support of the dead-center spring element on the side wall of the storage compartment and subsequently moves away therefrom. As a result, the dead-center spring element is initially tensioned and relaxes after the dead center has been overcome, wherein the dead-center spring element acts on the swiveling lid further in a direction of motion until a closed position or a usage position, in which the storage compartment is open and accessible, has been reached, and holds the swiveling lid in this position. One embodiment of the known storage compartment utilizes a compression spring in a telescopic tube, the spring being hinged to the side wall of the storage compartment and to the lever arm of the swiveling lid. Yet another embodiment utilizes a leg spring, the one leg of which is suspended on the lever arm of the swiveling lid and the other leg of which is suspended in the side wall of the storage compartment.
In known dead-center spring elements, the dead-center spring element is tensioned the most at dead center and the spring force of the dead-center spring element is greatest at dead center. It would be desirable to have, conversely, a low spring force at dead center and a high force in the closed position and/or in the usage position. As a result, the force necessary for opening and closing the movable part and for moving the movable part from a closed position into a usage position and vice versa, would be low in the vicinity of dead center, and a spring force, with the aid of which the dead-center spring element holds the movable part in the closed position and/or in the usage position, would be high. In the case of misuse, i.e., for example, in the case of forces in the opening direction that are greatly increased as compared to the forces necessary for a planned opening action, the known systems also have the problem that the opening and closing mechanism is destroyed. This relates to the dead-center spring element as well as to the relevant guides, joints, and stops.